In the recent years, energy saving has been demanded from the viewpoint of environmental problems, etc., and in order to achieve energy saving in a work machine such as a hydraulic excavator or wheel loader, it is important to save energy in the entire hydraulic system for driving the work machine. From the viewpoint of energy saving, a hydraulic closed-circuit system has been developed in which a hydraulic pump is connected in a closed loop to a hydraulic actuator to control the hydraulic actuator directly by the hydraulic pump.
Since the hydraulic closed-circuit system does not need a control valve which controls the supply direction and flow rate of hydraulic oil discharged from the hydraulic pump, no pressure loss attributable to the control valve occurs and it is only necessary to discharge hydraulic oil at a required flow rate from the hydraulic pump and there is little flow rate loss. In addition, the potential energy of the hydraulic actuator to be driven and the kinetic energy during deceleration can be regenerated so that energy saving can be achieved.
On the other hand, in the hydraulic closed-circuit system, in order for the amount of hydraulic oil discharged from a single hydraulic pump to cover the required amount of hydraulic oil to drive each hydraulic actuator, a large hydraulic pump with a high discharge rate is needed for each hydraulic actuator. Therefore, Patent Literature 1 discloses a conventional technique which converges flows of hydraulic oil discharged from a plurality of hydraulic pumps and achieves the drive speed of a hydraulic actuator without increasing the size of the hydraulic pumps. In Patent Literature 1, a hydraulic pump is allocated to each hydraulic actuator according to a priority order map which determines priority connection relationships between hydraulic pumps and hydraulic actuators and switching valves are controlled depending on this allocation.